A need exists for communication between personnel in large industrial and commercial facilities such as warehouses, power generating plants, refineries, manufacturing facilities, rail yards and sea ports, among other facilities. The use of radio communications is common; however, radio communications is not always effective in environments where the range of radio signals is limited by underground passageways, for example. Additionally, the ability to deliver emergency voice communications to a variety of personnel throughout a facility is difficult absent an accessible public address system.
One common system employs a page line and an audio line for station-to-station communications. In this manner, communications stations can be distributed throughout a facility and connected using page lines and audio party lines. Typically, only one conversation per party line can occur at a time, thereby limiting the number of simultaneous users to those who are parties to the conversation.
A user must first go off-hook to access the page line, page the desired party, tell the other party which alternate line to switch to, and release the page switch. Both parties must then switch to the same party line and go off-hook in order to converse. When the parties have completed their conversation, they must hang up the handset. Examples of known distributed audio systems are disclosed in U.S. Pat. No. 4,847,888 to Cox et al., U.S. Pat. No. 5,014,347 to Cox et al., U.S. Pat. No. 4,206,318 to Steely, U.S. Pat. No. 5,179,588 to Nowicki, U.S. Pat. No. 5,559,625 to Smith et al., and U.S. Pat. No. 5,033,080 to Deane.
For existing communications stations, on-hook/off-hook switching is accomplished using mechanical switches. For example, when the handset of a communications station is placed off-hook, a mechanical switch is activated to connect the telephone to the line. Such mechanical switches can be unreliable, large in size and expensive. Additionally, mechanical switches are generally exposed to environmental contaminants, for example, particulates or gases, which can corrode the mechanical switching mechanism and contacts. Other examples of harsh environmental conditions include exposure to moisture, corrosive vapors such as industrial chemicals or salt air, temperature extremes, particulates, radiation, electrical fields, stray magnetic fields, and the like. When exposed to these types of environments, the mechanical switches may corrode, become encrusted or otherwise become inoperable.
Alternatively, non-contact, magnetically-operated, reed switches have been used. The non-contact means are generally costly, more susceptible to stray electromagnetic fields, and less robust than mechanical switches. Reed switches, while generally sealed in an inert gas ampoule, may become inoperable when, for example, a fracture forms when exposed to temperature extremes, which may result in corrosion of the contacts. In another example, a stray magnetic field may cause the switch to become biased and therefore not respond properly.
Thus, there is a need for improved communications stations having reliable on-hook and off-hook switches that are small in size, low in cost and able to withstand harsh environmental conditions, and therefore are not susceptible to the corrosion of the contacts or weak stray magnetic fields.